The present invention relates generally to a system for purifying a liquid. In a preferred embodiment the invention specifically relates to an improved pressure and flow control system in a water purification system that uses reverse osmosis filtration.
Reverse osmosis (R.O.) filtration systems have been used to remove impurities from water. One end use for water purified in this manner has been rinse water in car washes. Removing impurities from the rinse water reduces spotting when the water dries, which is commonly referred to as a "spot free rinse (SFR)."
FIG. 1 shows the pertinent elements of a conventional SFR water purification system that is intended for applications requiring at least 500 gallons of filtered water per day. In this system feed water first passes through a prefilter 2. A valve 4 is provided to obtain samples of the feed water for evaluation. A solenoid valve 6 provides on/off control of the flow of feed water. A pump 8 circulates the water through the system. A switch 10 is provided to deactivate the pump if the pump inlet pressure is too low. The switch 10 also activates the pump, after a time delay, if the pump inlet pressure is high enough.
The pumped water is fed to a reverse osmosis filter cartridge 12. This filter removes impurities from the water with a reverse osmosis filter membrane (not shown). The water that passes through the membrane (i.e., the water from which impurities have been removed) flows out of a first outlet port 14 to a product water storage tank 16. Along the way, the product water passes a probe 18, a product water flow meter 20, and a spring loaded check valve 22. The product water probe 18 is available to evaluate characteristics of the product water. The product water in the storage tank 16 is available for its intended end use, which in this system is a spot free rinse application.
The pumped water that does not pass through the R.O. filter membrane exits the filter cartridge 12 through a second outlet port 24. This water passes by a pressure gauge 26, and then splits between two channels. Some of the water passes through a Paraplate backflow regulator valve 28 and returns directly to the pump 8. This water is recycled through the R.O. filter 12. The backflow regulator valve is an expensive component, and controls the water pressure in the system.
The remaining water exiting the second outlet port 24 passes through a WHITEY regulating brine valve 30, a drain water flow meter 31, and then to drain 32. This water then leaves the filtration system, and thus is referred to as "waste" water. The brine valve 30 controls the flow of water to the drain 32, and thereby contributes to the control of the velocity of water across the R.O. membrane.
A solenoid valve 34 is provided for flushing the R.O. filter membrane. This flush valve is closed during the previously described filtration operation. When it is open, however, most of the feed water passes through the second outlet port 24 and the flush valve 34, so that a higher, more turbulent flow is created along the R.O. membrane, thereby acting to cleanse the membrane.
Efficient filtration is achieved when the flow rates indicated by the product water flow meter 20 and the drain water flow meter 31 are the same. With these equal flow rates, the water recovery rate (i.e., amount of feed water that is purified to product water) is 50%. If recovery rate is too high, as indicated by a high product water flow rate, the R.O. membrane may foul quickly. This can happen if the brine valve 30 is accidentally closed. If the water recovery is too low, as indicated by a high drain water flow rate, the system is not operating efficiently and may not produce the desired gallons per day.
In order to obtain efficient filtration with the system shown in FIG. 1, the backflow regulator valve 28 and the brine valve 30 are adjusted until the flow rate in both the product water flow meter 20 and the drain water flow meter 31 are in the same range. At the same time, however, the water pressure indicated by the pressure gauge 26 should not exceed the maximum pressure recommended for the pump 8 (e.g., 200 psi). If the water pressure is too high, the pump will wear out quickly.
While the foregoing adjustments may appear to be simple, the proper adjustment can be very elusive because there is not a linear relationship between the two control valves 28 and 30 and the three key variables they affect (i.e., product water flow rate, drain water flow rate, and pressure). For example, the two flow rates could be drawing closer to each other and a slight turn of one valve sends the system pressure up to 250 psi. Turning the other valve may start widening the gap between the two flow rates. Obtaining the proper adjustment is more of an art than a science, and at times can be a tedious, time-consuming, frustrating effort. This is not a user friendly system. The problem is compounded by changes in the feed water (i.e., water temperature, TDS, etc.) which may require new adjustment to the two valves.